In a large-capacity rotating electrical machine, an armature winding is provided with upper and lower coil pieces arranged in two layers in slots provided in a laminated iron core, and the parallel circuits are connected in series, thereby increasing generated voltage and machine capacity. However, as a voltage of an armature winding is increased, the thickness of the main insulation of an armature winding is increased to withstand such higher voltage. As a result, a cross-sectional area of a conducting portion is decreased, a current density is increased, and a loss is increased.
Particularly, in a machine using an indirect cooling system, as the thickness of a main insulating layer increases, a thermal resistance increases, and a temperature increases in an armature winding. Therefore, an armature winding is divided into multiple parallel circuits, thereby decreasing the armature winding voltage and main insulation thickness, reducing the loss, and increasing the cooling capacity, while maintaining the machine capacity. Further, in an indirect-cooling large-capacity machine, it is common to increase the number of slots in an armature core to increase a cooling cycle of an armature winding. Therefore, an armature winding having three or more parallel circuits is required.
If an armature winding having three or more parallel circuits is applied to a 2-pole machine, the voltages generated by the parallel circuits are not completely equated, a circulation current is generated between the parallel circuits, and an armature winding loss is increased.
To decrease the loss by the circulation current, it is necessary to minimize the imbalance of the voltages generated by the parallel circuits. For this purpose, special consideration is required to placement of coils belonging to each parallel circuit in ach phase belt.
An example of improvement in placement of coils is explained with reference to a developed perspective view showing one phase of an armature winding shown in FIG. 30.
FIG. 30 shows an example of an armature winding having four parallel circuits applicable to a 3-phase 2-pole 72-slot rotating electrical machine, based on the U.S. Pat. No. 2,778,962 (hereinafter, US patent of Hamilton D. Taylor).
FIG. 30 shows only one phase. For the other two phases, it is obvious that the configuration of the shown phase of an armature winding is shifted by 120° and 240°.
In this patent, when parallel circuits are indicated by numbers 1 to 4, parallel circuits of twelve upper coil piece 15a and lower coil piece 16a of a first phase belt 17 are numbered 1, 2, 2, 1, 2, 1, 1, 2, 1, 2, 2, 1 sequentially from the center of a pole. Similarly, parallel circuits of upper coil piece 15b and lower coil piece 16b of a second phase belt 18 are numbered 3, 4, 4, 3, 4, 3, 3, 4, 3, 4, 4, 3 sequentially from the center of a pole. This decreases a deviation of voltage (an absolute value of a deviation from an average phase voltage) and a deviation of phase (a deviation of phase angle from an average phase voltage) of each parallel circuit.
To realize the above connection, in FIG. 30, fourteen jumper wires 20a per phase are provided at the connection side coil end 19a. 
As for the voltage and phase angle deviations of each parallel circuit, the U.S. Pat. No. 2,778,963 (hereinafter, US patent of Rudolph Habermann Jr.) is known.
In this patent, a voltage deviation of each parallel circuit is rated at 0.4% or lower, and a phase angle deviation is rated at 0.15° or lower. Compared with the voltage deviation of 0.12% and phase angle deviation of 0° in the US patent of Hamilton D. Taylor, the values are highly balanced, and sufficiently efficient to decrease a circulation current.
The connection method in the US patent of Hamilton D. Taylor provides an armature winding having four parallel circuits applicable to a 3-phase 2-pole 72-slot rotating electrical machine. In an indirect-cooling large-capacity rotating electrical machine, an armature winding having more parallel circuits is required.
The invention has been made considering the above circumstances. The invention may provide an armature winding having six parallel circuits applied to a large-capacity 3-phase 2-pole 72-slot rotating electrical machine.